Furnace and burner arrangement for heating steel slabs



J. D. WILDE Dec. 13, 1966 FURNACE AND BURNER ARRANGEMENT FOR HEATING STEEL SLABS Filed Sept. 11, 1964 4 SheetsSheet 1 JAMES B. WILDE ATTORN EY Dec. 13, 1966 J. D. WILDE 3,291,465

FURNACE AND BURNER ARRANGEMENT FOR HEATING STEEL SLABS Filed Sept. 11, 1964 4 Sheets-Sheet 2 ATTORNEY III! u-l E N 2 l E E s L:

\E E] E i "T32 l (2 E! T E 5 I I I E I INVENTOR JAMES I]. WILDE By I J. D. WILDE Dec. 13, 1966 FURNACE AND BURNER ARRANGEMENT FOR HEATING STEEL SLABS 4 Sheets-Sheet 3 Filed Sept. 11, 1964 mvamom JAMES B. WILDE gwy Wfl ATTORNEY Dec. 13, 1966 J. D. WILDE 3,291,465

FURNACE AND BURNER ARRANGEMENT FOR HEATING STEEL SLABS Filed Sept. 11, 1964 4 Sheets-Sheet 4 TEMP F RADIATKON ONLY TIME IN MINUTES 0 000 800 L200 L000 2000 ZLOO Z800 FURNACE TEMP IN "E INVENTOFL JAMES D. WILDE ATTOMIEI United States Patent M 3,291,465 FURNACE AND BURNER ARRANGEMENT FOR HEATING STEEL SLABS James D. Wilde, Weston, Ontario, Canada, assignor to Salem-Brosius (Canada) Limited, Rexdale, Ontario,

Canada, a corporation of Ontario Filed Sept. 11, 1964, Ser. No. 395,794 3 Claims. (Cl. 263-6) This invention relates in general to the heating of metal forms, and in particular, to a novel method and furnace arrangement for reheating steel to temperatures required for hot rolling.

Continuous furnaces for heating or reheating steel slabs and the like presently all operate on the principle whereby heat is applied to the furnace walls and radiated therefrom to the workpiece. Such furnaces, as for example the roller hearth slab reheating type, are generally from 300 feet to 400 feet in length and provide, at best, a rather slow heating of workpieces passed therethrough.

With the advent of continuous casting more rapid reheating is required of the subject furnaces if the latter are to be combined efiiciently with continuous casting plants, but to date, no adequate advance has been made in this field.

It has been generally accepted by experts in this art, that while heating by direct convection plays an important role up to temperatures of approximately 1800 F., above this figure, radiation heating alone is effective and convection is of little practical use. In particular, the concept of reheating large slabs of steel or the like, by direct convection, from 1800 F. to hot rolling temperatures, has been entirely discounted.

It is obvious that invthe absence of a more rapid heating expedient, the only measures to be taken to meet increased demands are twofold. Either larger and far more expensive furnaces must be substituted, or additional furnaces of similar dimensions to those existing must be entertained. In both cases, availability of space and capital outlay are most pressing problems.

The present invention avoids these problems by teaching an entirely new and novel concept as regards heating metal forms in furnaces of the general type hereinbefore noted. Use of this novel heating concept substantially increases the speed of reheating and thus decreases the necessary length of furnace structure required. Specifically, furnace length is decreased from the range of 300 to 400 feet to 200 to 250 feet and the rapid output of steel slabs by continuous casting methods and the like can be more adequately handled. Of course, it will be appreciated, as this disclosure proceeds, that conventional furnaces can readily be modified to incorporate the teachings of this invention and thus, further savings can be effected. I

This novel heating concept is directly contrary to the present thinking of those skilled in this field, teaching direct flame impingement on the metal slabs to effect rapid heating by convection from temperatures of 1800 F. up to required hot rolling temperatures.

It is therefore an object of the invention herein to provide a novel method of heating steel slabs to obtain a more rapid heat transfer rate than is possible with conventional methods presently in use.

It is also an object of this invention to provide a novel furnace and burner arrangement capable of carrying out the above noted method, and such a furnace, in taking advantage of the novel method, has resulted in structural advantages not obtainable with present methods which rely on radiant heat transfer alone.

Further objects and advantages will become apparent from the following detailed description taken in conjunction with the appended drawings in which:

3,291,465 Patented Dec. 13, 1966 FIG. 1 is a schematic cross-sectional view of the furnace heating zone,

FIG. 2 is a schematic cross-sectional view of the furnace soaking zone,

FIG. 3 is a plan view of the furnace,

FIG. 4 is an enlarged view of a portion of FIG. 3.

FIG. 5 is a side elevation of the furnace,

FIGS. 6, 6a and 6b are diagrammatic views of burner arrangements,

FIGS. 7 and 8 provide graphical comparisons between performance of applicants furnace and conventional radiation furnaces.

FIGS. 3, 4 and 5 of the drawings provide plan, side and detail views of a furnace of more or less conventional design and having the usual soaking and heating zones and flues.

In practicing the instant invention, no special materials or wall structure are required and the usual water cooled roller conveyors can be utilized as in known furnaces of this general type. Thus, existing furnaces may readily be converted or adapted to employ the novel features of the present invention.

Turning to FIG. 1, the schematic cross-section view of the heating zone in a furnace 10 is shown as having a plurality of high velocity burners 11 located in surrounding relationship to a conventional water-cooled roller conveyor 12 upon which the slabs 13 travel through the furnace when being heated.

The burners per se form no part of this invention and it is sufiicient to state that any burner capable of producing a nozzle or tile flame velocity of the order of at least 300 ft./sec. may be used.

The burners 11 are arranged in rows in the heating zone, two rows on each side of the median line of the roof 17 of the furnace in a staggered pattern, as shown in FIG. 4 to effectively cover the entire surface of the slab as hereinafter described.

As shown in FIG. 1, the burners in the heating zone are generally directed toward the longitudinal centre line of the furnace. The angle of inclination of the burners to the horizontal may vary from approximately 5 to but the preferred range is from 30 to 50 with optimum performance at approximately 35.

The distance from burner outlet to slab is quite critical and is limited to about 1 foot to 2 feet preferably 1'6 for the upper burners. The lower burners, in some furnace constructions, naturally cannot be situated as close to the slabs because of the conveyor rolls but these lower burner outlets should not be further than about 4 feet from the product being heated and should be so positioned as to preclude entry of falling foreign matter. The proximity and angle of the burners forms a highly important feature of this invention allowing high velocity flame impingement on the steel slabs and thus effecting heat trans fer by convection. Burner flame velocity may be varied as required to compensate for variation in the distance between the burner nozzles and the slabs or workpieces. Naturally the furnace will also heat by radiation in known manner.

It will be appreciated that the flames effect a washing action on the steel and remove or inhibit scale formations of a loose nature and further removes the well known protective air film thus facilitating heat transfer. Preferably, the burners of each pair are directly opposed to each other although satisfactory results can be obtained with an offset relationship. It is noted that the burners may also be positioned to wash the slabs longitudinally of the heating zone as well as, or instead of, transversely. Further, it is feasible to locate burners in the roof of the furnace alone, this latter expedient being of considerable use in the conversion of existing conventional furnaces where difliculty may be encountered in mounting the lower burners.

In actual tests conducted with laterally opposed sets of burners it was found that best results were attained when the flame-jets of the upper burners intersected at the upper surface and centrally of the slabs while the lower jets intersected on the lower surface and centrally of the slabs. The outer pairs of upper burners direct jets onto the slabs outwardly of the point of intersection of the inner jets and are positioned in staggered relationship longitudinal as shown generally at 15 in FIG. 4.

The soaking zone in FIG. 2 is of more or less conventional design including vertically positioned upper burners 14 installed for radiation heating although, the high velocity burners can be used here and such are shown installed in the lower part of the furnace. In short, conventional radiation heating may be used in those areas of the furnace where the surface of the workpiece is at correct rolling temperature to avoid overheating.

To establish the best burner arrangement, several different arrangements were tested on a heat exchanger 18 and the best arrangement of those tests is the arrangement diagrammatically illustrated in FIGS. 6, 6a and 6b. FIGS. 6a and 6b are plan views of FIG. 6 showing opposite and staggered burner arrangements. In carrying out the test the following general procedure was utilized for the purpose of accumulating data for testing each arrangement.

The furnace was fired for two days before data was recorded to thoroughly dry the refractory lining. Furnace temperatures were determined with an optical pyrometer at seven different locations inside the furnace and the arithmetic average furnace temperatures were calculated from that data. Furnace outside shell temperatures were measured with Tempilstiks. Water temperaures at the entrance and exit of the heat exchangers were measured with calibrated well-type mercury thermometers. The water piping on the exit side was insulated to the thermometer locations. The water temperature rise across each flow channel was measured. The water flow rate was determined by the weight/time method. The flow rate was measured for each individual channel, and measurements were required periodically to ensure against changing water flow rates.

The burners were constantly fired at capacity, i.e., 1 mm. B.t.u./ hr./ burner with approximately 5% excess air. The velocity of the products leaving each burner tunnel was constant for all tests. The fuel used was natural gas.

The data was recorded periodically throughout the tests so that information could be obtained for various furnace temperatures. The result of the tests established the combined heat transfer rates (radiation plus convection) for each burner arrangement. The results of the tests which were carried out indicated that the increase in heat transfer rate was due entirely to convection, or flame heating, and is dependent upon the physical arrangement of the furnace and not upon the furnace temperature. This statement is limited by the fact that the quantity and mass-flow rate of the combustion products leaving each burner were held constant for all tests. The greatest gain in heat transfer rate by convection was obtained with the burners arranged according to FIGS. 6, 6a and 6b. With this arrangement, the gain due to convection was approximately 40% of the total heat transferred by convection and radiation at 2300 F. furnace temperature.

Turning to FIGS. 7 and 8, the results of the tests are shown in graph form.

FIG. 7 is a time-temperature plot and clearly illustrates the substantial saving in heating time realized by the use of the present invention as compared with the conventional radiation furnace. Identical steel slabs, 5" thick, of low carbon steel took approximately five minutes longer to reach hot-rolling temperature in a known furnace under radiation alone than with combined radiation-convection heating as effected by the novel method and furnace arrangement set forth herein.

FIG. 8 is a plot of heat transfer rate v. furnace temperature. This graph demonstrates the substantial increase in heat transfer rate through using the combined effect of radiation and convection.

For example, at a furnace temperature of 2300 F., heat transfer due to radiation alone is approximately 54,000 B.t.u. ft. /hr. while at the same temperature, the heat transfer rate with combined radiation and convection is approximately 97,500 B.t.u./ftF/hr.

In operation, the heating zone is maintained at a temperature approximating 2700 F. and the soaking zone at approximately 2350" F. or 2400 F. Upon variation of the control temperature the fuel input can be regulated in known manner to stabilize the temperature at the preferred level.

Slabs may be fed into the furnace either hot or cold and in passing through the heating zone are subjected to direct flame impingement or direct convective heating plus radiation heating.

A comparison of applicants method and the conventional radiation method (see FIGS. 7 and 8) shows that given test pieces required approximately 15% to 17% more heating time to reach required temperature in a conventional furnace than in applicants. Relating this invention to furnace length and considering the very high cost per foot of these furnaces, it will readily be appreciated that applicants method and arrangement of high velocity burners for carrying out that method marks a very significant advance over conventional practice.

To those skilled in the art there are many variations possible within the scope and spirit of this invention and it should be noted that this disclosure should be construed in an illustrative rather than a limiting sense.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A continuous reheating furnace including a plurality of laterally opposed high velocity burners spaced along the length of the furnace and grouped in surrounding relationship to conveying means therein; said burners being generally directed towards the longitudinal centre line of said furnace; said plurality of high velocity burners including:

(i) a pair of parallel, longitudinally extending upper rows, each burner in one row of said pair lying on a common transverse line with a corresponding burner in the other row of said pair of rows,

(ii) a lower pair of burner rows below the conveying means,

(iii) an outer pair of parallel, longitudinally extending upper rows in longitudinally staggered relationship to the first mentioned upper pair of burner rows,

to effect direct flame impingement on work-pieces carried by said conveying means.

2. A reheating furnace as defined in claim 1 wherein the burners are inclined to the horizontal at an angle in the range between 5 and 3. A reheating furnace as defined in claim 1 wherein the burners are inclined to the horizontal at an angle in the range between 30 and 50.

References Cited by the Examiner I UNITED STATES PATENTS 2,529,690 11/1950 Hess 2636 2,762,618 9/1956 Johnson et a1. 2636 FREDERICK L. MATTESON, IR., Primary Examiner.

JOHN J. CAMBY, Examiner. 

1. A CONTINUOUS REHEATING FURNACE INCLUDING A PLURALITY OF LATERALLY OPPOSED HIGH VELOCITY BURNERS SPACED ALONG THE LENGTH OF THE FURNACE AND GROUPED IN SURROUNDING RELATIONSHIP TO CONVEYING MEANS THEREIN; SAID BURNERS BEING GENERALLY DIRECTED TOWARDS THE LONGITUDINAL CENTRE LINE OF SAID FURNACE; SAID PLURALITY OF HIGH VELOCITY BURNERS INCLUDING: (I) A PAIR OF PARALLEL, LONGITUDINALLY EXTENDING UPPER ROWS, EACH BURNER IN ONE ROW OF SAID PAIR LYING ON A COMMON TRANSVERSE LINE WITH A CORRESPONDING BURNER IN THE OTHER ROW OF SAID PAIR OF ROWS, (II) A LOWER PAIR OF BURNER ROWS BELOW THE CONVEYING MEANS, (III) AN OUTER PAIR OF PARALLEL, LONGITUDINALLY EXTENDING UPPER ROWS IN LONGITUDINALLY STAGGERED RELATIONSHIP TO THE FIRST MENTIONED UPPER PAIR OF BURNER ROWS, TO EFFECT DIRECT FLAME IMPINGEMENT ON WORK-PIECE CARRIED BY SAID CONVEYING MEANS. 